Thus the NA and Centre of Gravity of Compressive Forces further shifts
upwards to maintain Equilibrium.
This results in an increase in the Moment of Resistance of the Beam. This
process of shift in the NA continues until maximum strain reaches its Ultimate
Value i.e. 0.35% and the Concrete Crushes.
Such a beam is referred to as "Under Reinforced Beam".
The Failure is called Tension Failure because Yielding of Steel was responsible
for higher strains in concrete resulting in its failure.
Over Reinforced Beam :-
When the amount of steel is kept more than that in the Balanced, the NA tends
to move downwards and the Strain in Steel remains in Elastic Region.
If the beam is further loaded the stresses in steel keeps on increasing and so the
force of tension.
Here the force of tension is more than that of compression, and hence to
maintain the equilibrium of tensile and compressive forces the area of concrete
resisting compression has to increase so as to increase the force of compression.
In this process the NA furher shifts downwards until maximum strain in
concrete reaches its ultimate value of 0.35% and concrete crushes. The Steel is
well within Elastic Limits.
Such a beam is referred to as an "Over Reinforced Beam" and the failure as
Compression Failure